1. Field of Invention
The invention relates to a texture mapping process for 3D models for mapping images onto a 3D model. In particular, the invention is a multilevel texture processing method that processes images at the image level, the texture level, and then the pixel level and adjusts image pixel intensity on the texture.
2. Related Art
The texture mapping technology is used to increase the reality of computer generated images (CGIs) in the field of computer graphics. Through realistic images, the texture mapping technique provides better image rendering effects with fewer polygons of the 3D models. With the advance in integrated circuit (IC) designs, the operating functions and texture memory required for texture mapping have been incorporated into most of the graphic acceleration chips. Therefore, one can find many examples that use texture mapping to generate photo-realistic images in different kinds of applications, such as games, animations, and 3D websites.
Generally speaking, to obtain a textured 3D model, one has to construct a complete and accurate digital 3D model of the object manually or by 3D scanning. The next step is to take 2D images of the object and to establish projection relations between the 3D model and the 2D images. After obtaining the 3D model of the object and the projection relation between it and some perspective image, we can project the digital 3D model onto a 2D image through the projection relation. In this manner, the image areas covered by the polygons in the 3D model are set to be the textures corresponding to the polygons. This completes the settings of the texture mapping.
When mapping textures from an image onto a 3D model, we can use the above-mentioned procedure to obtain a textured model rapidly. However, an image is not sufficient to cover the textures of the whole surface of the object. In this case, the rendering of the model has a crack or a shading problem. Consequently, one needs images from different perspectives to completely determine the texture for each polygon of the 3D model. When taking images of an object from different viewing perspectives, there may be different shading for the same point on the object surface due to the facts that pictures are taken at different time, different light sources, and under different camera setting conditions (zooming, focus, camera position). Such differences will result in variations in the texture of a digital 3D model, and show visual defects in the image rendering.
To eliminate this problem of texture inconsistency, we can generate a global map from a textured 3D model through projections, such as projecting textures of the 3D model onto a cylindrical or spherical surface. The pixel color variations in each area are then adjusted manually using existing image processing software such as PhotoImpact© and PhotoShop©. However, manually adjusting the intensity of each pixel on the global map requires a lot of time for a person familiar with image processing tools. In U.S. Pat. No. 6,057,850, a method has been proposed using a calibrated lighting system that can be controlled and positioned to take images of an object with different lighting conditions. The images are stitched together by performing different levels of intelligent pixel-wise blending according to the lighting condition and the camera position. In this manner, shades due to different lighting conditions are eliminated by averaging according to their weights, providing an image with less dependence on the light sources. Afterwards, textures are extracted from the image and mapped onto the 3D model. This method allows one to rebuild a new image using different lighting conditions and camera positions. Nevertheless, it requires a special lighting system with the positioning function to obtain a better effect.
On the other hand, the U.S. Pat. No. 6,281,904 divides the 3D model into several different planar areas. Firstly, the user manually selects a corresponding projection region in an image for each region on the 3D model. The projection regions for the same region on the 3D model on different images are mixed together using various methods. This method is concerned with how to modify the texture resolution and intensity of the same region from different images but it does not smooth the differences among different regions. Also, occlusion occurs for the common region in different images is not dealt with. To improve the situation, the U.S. Pat. No. 6,469,710 proposes the concept of alpha blending. The contribution of each pixel in the projection regions of different images for the same 3D model surface region to the final texture is determined. If there are images of other objects in the projection region, the contribution of this pixel is set to be zero, eliminating effects of erroneous pixels. However, the texture processing operation is applied to the projection of the same 3D model surface region individually. The texture variation among regions will still have different visual transition effects.